Up till now, image processing apparatuses or methods for detecting specific shapes from picked-up images are widely used. For example, in image processing apparatuses for FA (Factory Automation), detection of misselection of parts and detection of defects in partial shapes are realized by detecting the partial shapes. Further, detection of specific shapes is used in recent robot navigations to recognize leading signs and recognize buildings by remote sensing. Further, to assist driving, there are apparatuses that recognize traffic signs to notify the driver of information about the recognized traffic signs or use the information about the traffic signs for vehicle control. A sign recognition apparatus recognizes traffic signs by picking up an image in the front by a camera, extracting the image of a specific area from the picked-up image according to, for example, the shapes used for traffic signs, and by performing pattern recognition such as template matching between the extracted image and a template image of traffic signs.
In shape detection, a conventional Hough transform (see Patent Document 1) is used. Here, voting is performed for parameters of analytic formula that describes the shape by edge points of the image to calculate the parameters of the analytic formula for the shape from the peak values in the parameter space. The standard Hough transform method is used for only the shape represented by the analytic formula such as straight lines and circles. By contrast with this, the generalized Hough transform method (see Non-Patent Document 1) can detect arbitrary shapes. In the generalized Hough transform method, a reference point is designated, and the association relationships between features of the points of a shape and the geometrical relationships between the points and the reference point, are prepared in advance by a standard template for the shape. There are generally a plurality of the above-noted association relationships, and, consequently, to detect an image, voting is performed for all of the possible association relationships from the edge points of the image. From the voting result, the association relationship of high possibility is found and the target shape is detected. Further, to realize improved processing speed of the Hough transform and generalized Hough transform, for example, a technique of limiting the number of voting points is employed (see Patent Document 2).
Further, a technique of detecting regular polygons using a voting method different from the Hough transform is suggested (see Patent Document 3). In this technique, voting is performed for the center of a shape having edge points in radial symmetry, and, consequently, the voting space is actually the same as the image space. Therefore, the processing speed is higher than the conventional Hough transform.
Patent Document 1: U.S. Patent Application Laid-open No. 3069654
Patent Document 2: U.S. Patent Application Laid-open No. 2006/0291726
Patent Document 3: U.S. Patent Application Laid-open No. 2006/0098877
Non-Patent Document 1: D. H. Ballard, “Generalizing the Hough transform to detect arbitrary shapes, Pattern Recognition,” Vol. 13, No. 2, 1981